TRANSACTIONS OF THE SECTIONS. 189 



the dimensions proposed by the writer for the Atlantic, having a diameter off inch, 

 bears between 3 and 4 tons of longitudinal strain before breaking, or fully as much 

 as an iron-covered cable of the same diameter, while its weight in sea- water is from 

 five to six times less. 



The machine used in the manufacture of this cable presents several practical 

 mechanical combinations, which the writer, however, passed over, in order not 

 to extend the subject of this paper. 



The peculiar properties of this cable, as contrasting with the ordinary cable with 

 helical or spiral sheathing, are as follows : — 



1st. The absolute strength of this cable can be increased to any desired extent 

 without adding to its weight in water, the hemp being very nearly of the 

 same specific gravity as the water itself. Considering past experience, a 

 specific gravity of 1.}, and a strength sufficient to support from ten to twelve 

 miles of weight of the cable in sea-water appears desirable. 

 2ndly. The elongation of this cable under half its breaking strain does not exceed 

 -!■ per cent., and has no tendency to untwist when suspended from one end, 

 whereas an iron-sheathed cable (or a cable of the Toulon and Algiers type) 

 will elongate from 3 to 4 per cent, under similar circumstances, and partially 

 untwist at the same time, because each wire tends to assume a straight line 

 parallel to the axis. This elongation throws a great and dangerous strain upon 

 the insulated conductor, which will be permanently elongated and disposed to 

 fall into kinks at the bottom. 

 3rdly. In the manufacture of iron-clad cables the insidated conductor frequently 

 gets injured through the breakage of a wire, or the carelessness of workmen 

 in making welds, and the cable, when manufactured and in the very act of 

 being paid overboard, may be penetrated by the sharp end of a broken wire, 

 as has most unfortunately happened on a recent occasion. The copper-sheathed 

 cable precludes almost the possibility of any such accidental injury ; the in- 

 sulated conductor, after being carefully tested, passes through the sheathing- 

 machine, where it receives spontaneously its double hemp covering and 

 complete metallic armour coat. It is worthy of remark that the insulation 

 resistance of the cores invariably increases in receiving the armour coat, 

 owing, it appears, to the external pressure produced, and that not a single, 

 fault of insulation has ever occurred in the cable during the processes of ship- 

 ment and submersion. 

 4thly. The durability of the copper-sheathed cable has already been proved by 

 experiment and in actual practice. A cable which had been laid in 1864 in 

 the Mediterranean, from Oran to Carthagena, was raised again nine months 

 later (in consequence of an accident of a purely mechanical nature) from a 

 depth of 1500 fathoms, when it was found to be in a perfectly good condition, 

 the insulation and the strength of the hemp being unimpaired, and the copper 

 sheathing being covered by a thin green scale of chloride of copper and mag- 

 nesia, it is supposed, which appears to have arrested further chemical action. 

 The cable so recovered, and of which a specimen was exhibited, now forms part 

 of the line connecting Sicily with Algeria ; whereas the iron-sheathed shore- 

 end cable, which was also recovered, was found to be unfit for further use, being 

 much corroded and covered with coral growth. 

 5thly. The copper-sheathed cable is mechanically well suited to the operations 

 of laving; it is extremely flexible, occupies relatrvely little space in the ship's 

 holdj! and glides smoothly overboard without the least risk of catching by 

 broken wires, or of forming into kinks, or untwisting under strain. It offers 

 remarkably little resistance in the water, and can be recovered from the 

 greatest depth at the rate of one knot per hour. 

 It has been objected that hemp rope contracts very considerably (about 3 per 

 cent.) upon being immersed, and that the copper-sheathed cable must be liable to 

 the same injurious effect. While fully admitting the general proposition, the 

 writer has no difficulty to show that it does not apply to the cable in question. 



It was seen from the experiment which was shown to the Section, that although 

 a twisted hemp rope contracts upon being moistened, a single hemp fibre, taken 

 from the same rope, actually elongates under the same treatment, the reason 



